1. Field of the Invention
This invention relates to a scanning optical system which eliminates pitch unevenness of scanning lines.
2. Description of the Prior Art
In the art of light beam scanning devices using a deflecting and reflecting surface such as a rotating polygonal mirror, various scanning optical systems have been proposed in which no unevenness is created in the pitch of scanning lines on a surface to be scanned (a medium to be scanned) even if the direction of travel of a light beam deflected and scanned is varied in a plane perpendicular to the deflection plane by the tilting of the deflecting and reflecting surface of the mirror. The term "deflection plane" used herein refers to a light beam plane which is formed with lapse of time by a light beam deflected by the deflecting and reflecting surface of the deflector.
For example, in U.S. Pat. No. 3,750,189, the optical system between a deflector and a medium to be scanned comprises beam reforming means and second converging means. A light beam reflected by a deflecting mirror is collimated by the beam reforming means. However, when the optical system has the collimating function, limitations are imposed on the configuration of the beam reforming means, and the imaging performance on the surface to be scanned and the degree of freedom which will better the strain characteristic for making the scanning speed constant will be decreased. Unless the number of lenses forming the second converging means is increased, good performance cannot be obtained.
In U.S. Pat. No. 3,865,465, a predetermined limitation is imposed on the ratio of the focal lengths of two lenses forming the optical system between the deflector and the medium to be scanned and satisfying this limitation is equivalent to collimating the light beam in a cross section perpendicular to the deflection plane between the two lenses. Accordingly, again in this example, the image performance and the degree of freedom which well corrects the strain characteristic are decreased and this is not desirable.
In U.S. Pat. No. 3,946,150, a cylindrical lens is disposed between a lens having a strain characteristic for realizing uniform speed scanning and the medium to be scanned. In the case of such construction, a good image cannot be obtained unless the position of the cylindrical lens is brought close to the medium to be scanned. If the cylindrical lens is brought close to the medium to be scanned, it will become longer in the direction of the bus line as the scanning width becomes greater, and this will prevent the construction from being made in compact form.
Further, in an optical system using a cylindrical lens or a toric lens in which, as disclosed in the aforementioned U.S. Pat. No. 3,750,189, the refractive power of the optical system differs in a direction in which the beam is deflected by the deflector and in a direction orthogonal to that direction, if a machining error or error in assembling the lenses exists, then a difference in the imaging point between the two orthogonal directions will result. This is the so-called astigmatic difference and, when such astigmatic difference is produced, the optimal imaging point cannot be obtained even if the position of the medium to be scanned is adjusted.